Conventional wire electrical discharge machining (EDM) equipment includes a head assembly mounted on a machine tool body and a numerically controlled positioning table. The wire equipment contains a spool assembly on which a fine wire is held taut between two spools. The drive spool moves the wire at a fixed rate from one spool to another with the useable work length between the two spools being about 4" to 8", depending on the work height to be cut. The cutting rate depends upon the wire size and the amperage carried by the wire, plus the feed rate set for the depth of the cut. Deionized water is not only normally used as the dielectric, but also functions to flush out eroded particles produced during the cutting action, however, hydrocarbon oils can also be used. As is typical with all EDM operations, the wire electrode may produce a uniform overcut or overburn of about 0.002" to 0.003" over the diameter of the wire electrode.
In the extrusion forming of ceramic honeycomb structures used in catalytic converters for internal combustion engines, it is necessary to form extrusion dies with substantially long, thin slots which are not only straight and extend across the face of the die, but which also have parallel sidewalls, are burr free and have consistent surface finish. In addition, such long thin slots have a length greater than about 31/2", a depth in excess of about 0.1" and a width not greater than about 0.012".
Known EDM wire processes, such as disclosed in U.S. Pat. Nos. 2,526,423; 4,205,213; 4,233,486; 4,403,131; and 4,527,035 are all capable of producing slots, however, such processes are plagued with the problem of forming non-uniform slot widths along the length of the formed slots, which problem is virtually inherent in the process. That is, measurements have indicated that an EDM wire-cut slot varies in width from the point of wire entry to the point of wire exit, with the width of the entry point always being wider than the width of the exit point. As the strand of wire travels through the slot, it is constantly being eroded, thus becoming smaller in cross-sectional area as it nears the exit point of the workpiece. Additionally, the wire tends to vibrate, similar to a guitar string, with the highest amplitude being near the longitudinal center of the slot being cut within a workpiece, forming a wide center region.
Thus, a tapered pattern is formed along the longitudinal extent of each slot, with a medium slot width being formed adjacent the wire entry point of the workpiece, a widest slot region being formed centrally of the longitudinal extent of the slot being formed, and a narrow slot width being formed adjacent the wire exit point of the workpiece. This tapered width pattern repeats itself each time a slot is formed across a workpiece from side to side, and from end to end when cross slots are formed. In the case of rows of slots formed in the discharge face of an extrusion die for forming cellular structures, the effect of this variation in slot width across each of the slots creates a graded condition across the die which correlates very closely with a problem of bow in the extrudate. The formation of bow, which is the tendency for the extrudate to curve upon leaving the extruder, is an undesirable condition and produces detrimental effects in the formation of cellular or honeycomb structures.
The formation of bow is not a self-correcting disturbance during the extrusion process. That is, the faster flowing areas of the die will continue to flow faster unless some sort of corrective hardware is utilized. A bow deflector is the most commonly used state of the art item for correcting bow, but like most other corrective hardware devices, it is viable only for very modest degrees of bow. At best, such corrective artifices represent crutches, which simply apply an opposite unbalancing effect to offset the original unbalanced condition and in some cases other flow problems, all in the hope of combining two asymmetrical conditions into an acceptable one.
The present invention overcomes the problems of the slot width variation encountered with known EDM wire processes by correcting or greatly ameliorating the bow problem within an EDM slotted die, and thus virtually eliminating the need for extraneous remedial hardware previously necessary to correct such problem.